The present invention relates to a control system for a vehicular braking system incorporating a hydrodynamic brake and a friction brake, in particular for heavy-duty wheeled and tracked vehicles, wherein as a measure of a predetermined total brake torque the brake pedal travel, or the brake pedal force is utilized and the relevant total brake torque is generated by the hydrodynamic brake with first priority, and wherein the friction brake is controlled such that it generates the difference between the predetermined total brake torque and the brake torque provided by the hydrodynamic brake.
Control systems of this type are known. The purpose of such control systems is to ensure that the friction brake is required to provide only that share of the total braking torque that cannot be provided by the hydrodynamic brake as a result of its torque loss in the lower speed range. The passing of braking action from the hydrodynamic brake over to the friction brake should be performed with a minimum possible drop in the total brake torque.
From German Pat. No. DE-OS 2,120,743, a control system for a vehicular braking system incorporating a hydrodynamic brake and a friction brake is known, wherein the total brake torque to be generated is predetermined by the brake pedal travel, wherein the brake torque is generated with first priority by the hydrodynamic brake by means of a controlling means, and wherein the friction brake is controlled such that it generates the difference between the predetermined total brake torque and the brake torque provided by the hydrodynamic brake. The controlling means essentially comprises a pressure control valve serving to control the application of the friction brake and influenced by the predetermined desired value of the total brake torque and at the same time by a control pressure which is proportional to the brake torque of the hydrodynamic brake and counteracts the predetermined desired value in this pressure-control valve.
The main disadvantages of such a hydraulic system are (1) the delays in pressure build-up occurring in the long lines, in particular in case of air inclusions, (2) a too rapid response of the friction brake relative to the hydrodynamic brake at higher speeds, and (3) the uncertainty of the feedback signal for the brake torque of the hydrodynamic brake.
Another control system of the above-mentioned type is disclosed in U.S. Pat. No. 4,418,963, assigned to the same assignee as the present application, whose disclosure is incorporated herein by reference. This control system overcomes to a certain extent the above-mentioned disadvantages.